PSL is a common acronym for personal seat license. As far as I can tell, a personal seat license is a fee you pay to a club or other sports entity to gain the right to then buy season tickets.
PSLs were developed sometime in the mid-to-late 1990s as a method of partially funding stadium building costs. The Baltimore Ravens, for example, required fans to buy a PSL before buying season tickets for many of the seats at the Camden Yards stadium (currently PSINet Stadium).
In the case of the Ravens, above, tickets still sold - 50000 season tickets sold in the two weeks following their first offer, even with the PSL fee added on top in most cases. This success and others led many other stadium construction projects to adopt at least the part of the financing model involving these personal seat licenses.
A cursory glance at current building projects show that a variety of sports arena and stadium projects use this model - college football, professional hockey, basketball, both professional and collegiate, and so on.
Any information you have on the first innovation of PSLs is welcome; any search I do on google just brings up lots of places trying to sell them.